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Machines in External

Beam Radiotherapy

(EBRT)

Dr. Anushree Punia

➢ EBRT – Radiation is delivered from a source

which is at a certain distance from the patient

Radiation Therapy Equipment

On the basis of radiation energy used EBRT can be

classified as follows:-

1. Grenz Therapy

2. Contact Therapy

3. Superficial Therapy

4. Orthovoltage Therapy

5. SuperVoltage Therapy

1. Grenz Therapy

➢ Has a very soft beam (<20kV).

Absorbed within the first 2 mm

of skin - does not penetrate

beneath the dermis.

➢ Used clinically for skin lesions

like Atopic Dermatitis,

Psoriasis, Lichen Planus, Acne

➢ 40-50 kV small focal spot, short SSD (~5

cm)

➢ Absorbed with 2 cm of tissue.

➢ used for endocavitary irradiation.

Selected Rectal, Oral Kaposi Sarcoma etc)

2. Contact Therapy

3. Superficial Therapy

➢ 50-150 kV. SSD: 15/20 cm.

➢ The beam energy penetrates only the top

surface layer of the skin

➢ Treatment option for skin tumors of 5.0 mm

depth including BCC, SCC or Kaposi’s

sarcoma.

Grenz

Superficial

4. Orthovoltage (Deep)

Therapy➢ 150 - 500 kV.

➢ Treatment fields used to be defined using detachable cones.

➢ The SSD was typically 50 cm.

➢ Not in 'clinical' use- high skin dose

5. Supervoltage Therapy➢ 500-1,000kV

➢ Due to the demand of treating deeper tumors these units were created.

➢ Since conventional power transformers were not suitable for high energy

units (>300kV) so for new machines resonant Transformers were created to

facilitate which step up the voltage in an efficient manner

6. Megavoltage Therapy

➢ X-rays beam of energy 1 MV or greater.

➢ Teletherapy – Co60

➢ Particle accelerators - A device that uses

electromagnetic fields to propel charged

particles to high speeds and to contain them in

well defined beams.

TYPES

- Van de Graaff generator

- Betatron

- Linear accelerator

Click icon to add pictureBryant Symons

radium "bomb"

at Westminster

Hospital,

London,

England, in the

1930s.

➢ Until 1951, all isotope machines produced were teleradium units

(radium bomb).

➢ The SSD was usually not greater than 10 cm in these machines.

➢ Major drawbacks of these machines were high risk of radiation hazard

due to radon gas leak produced as a by product, high cost of radium,

large self absorption, low γ ray constant and low output

➢ For telecaesium units the SSD is 20 - 40cm.

➢ They have not been very popular because of relatively low γ ray

constant and low specific activity.

Isotope Machines

Telecaesium Unit

with applicators

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➢ 60Co teletherapy unit invented by

H.E. Johns in Canada in the early

1950s

➢ The first two cobalt teletherapy units

were installed in Canada in 1951, at

the Saskatoon Cancer Clinic and the

Victoria Hospital, London Ontario.

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Cobalt MachinesCobalt Machine

COLLIMATOR

COUCH

BEAMSTOPPER

GANTRY HEAD MAIN FRAME

The main components of a

teletherapy machine are: ➢ a radioactive source

➢ a source housing, including beam collimator and source

movement mechanism;

➢ a gantry and stand

➢ a patient support assembly; and

➢ a machine console.

.

Radionuclides have been used as source of γrays for teletherapy

Radium-226, Cesium-137, Cobalt-60

60Co has proved to be most suitable for external beam R/T

Higher possible specific activity

Greater radiation output

Higher average photon energy

RadionuclideHalf-Life

(Years)

γRay Energy

MeVSpecific Activity Achieved in

Practice (Ci/g)

Radium-226 (filtered by 0.5 mm Pt)

1622 0.83 (avg.) ~ 0.98

Cesium-137 30.0 0.66 ~ 50

Cobalt-60 5.26 1.17, 1.33 ~ 200

Source

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60Co Source

The overall diameter of

a 60Co source is 1 to 3 centimeters.

Consists of pellets of radioactive

60Co encased in multiple layers of welded metal to prevent contamination of the environment and to absorb β-particles produced by the decay process.

Or 60Co sources are made with the 60Co fused into a solid cylinder. Advantages:

Smaller source with less penumbra for the same beam intensity

Less hazard of contamination should a source

Typical source activity: of the order of 5 000

- 10 000 Ci

Typical dose rates at 80 cm from source: of

the order of 100 - 200 cGy/min

Teletherapy source is usually replaced within

one half-life after it is installed.

❑ methods for moving the source from OFF position to ON

position-

1. Source mounted on a rotating wheel inside the source

head to carry the source from OFF to On position

1. Source mounted on a heavy metal drawer is moved

horizontally by pneumatic system 3. Mercury is allowed to flow into a container immediately

below the source to shut OFF the beam.

4. Source is fixed in front of the aperture and the beam can

be turned ON and OFF by a shutter consisting of heavy metal

jaws.

Collimators➢ Collimators provide beams of desired shape and

size.

➢ Collimators of teletherapy machines provide

radiation fields typically ranging from 5 × 5 to 35

× 35 cm2 at 80 cm from the source.

➢ The rotational movement of the collimator is

continuous and it can rotate 360° about its own

axis.

Gantry

➢ The gantry can rotate by 360°.

➢ The rotational movement of the gantry is motorized

and controlled in two directions continuously; its

rotation speed can be adjusted.

➢ Teletherapy machines are most often mounted

isocentrically, allowing the beam to rotate about

the patient at a fixed SAD.

➢ They can be used either as fixed field machines or

rotation units.

➢ Modern teletherapy machines have SADs of 80

The axis of rotation of the three structures: Gantry, Collimator and Couch

coincide at a point known as the Isocenter.

Isocentric Mounting advantages:-

➢ Enhances accuracy.

➢ Allows faster setup and is more accurate than older non isocentrically

mounted machines.

➢ Makes setup transfer easy from the simulator to the treatment machine.

Isocenter

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Patient Support Assembly

Treatment Bed has horizontal, vertical and lateral

motorized movements

➢ Table Top - 90o rotation to each side

➢ Base - 110o rotation to each side

➢ Bedtop size l x w cm (in) - 235 x 46 (93x

18)

➢ Patient weight capacity kg (lb) - 136 (300)

Control Console

➢ Control Console is situated outside the bunker

➢ It consists of following:-

CONTROL PANEL - The control panel including following functions..Confirm

push button, Treat push button, Pause push button, Terminate push button,

Power on indicator, Inhibit indicator, Inhibit reset key, Power key switches,

Audible signal, Auto set up /motion enable push button, Remote gantry

positions

DISPLAY MONITOR - A display monitor displays system information such

as..Start up information, Treatment time, Wedge details, Field size and

collimator angle, Table parameters

Beam characteristics for 60Co

1. Depth of maximum dose = 0.5 cm

2. SSD = 80cm

3. Increased penetration (10-cm PDD = 55%)

4. Beam edge not as well defined—penumbra due to

source size

5. Dose outside beam low because most scattering is

in

forward direction

6. Isodose curvature increases as the field size

❑SAFETY &PROTECTIVE FEATURES

❑EMERGENCY SWITCHES

➢ The emergency switches when pressed this will

remove the power from the unit and table motion

drive circuit & also source goes to its fully shielded

position

They are located on

➢ Control console

➢ Main frame of machine

➢ Hand control

MACHINE

NAME 780 780 C ELITE 80 EQUINOX

FIELD SIZE Min:5X5;

Max:35X35

Min:5X5;

max35X35

Min:5X5

Max:35X35

Min:4X4

Max35X35

JAWS SYMMETRIC SYMMETRIC SYMMETRIC ASYMMETRIC

WEDGE 30o 45o

60o

Very rarely

used

30o 45o

60o

Very rarely

used

15o 30o

45o 60o

Max field

size:

15W x 20

For 60 :10W

x15

15o 30o 45o

60o

Max field size:

15W x 20

For 60: 10W x

15

BREAST CONE NO YES YES NO

TRIMMER YES NO No NO

BLOCK AP Y Y Y Y

PA Y Y Y NO

LATERAL NO Y Y NO

Bhabhatron - I

An Indigenous Teletherapy cobalt machine

developed in INDIA

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Bhabhatron - II Panacea Medical Technologies Pvt LTD

Bhabhatron - II

MAJOR FATURES :

Indigenous Teletherapy Machine

Low Cost

Battery backup

Enhanced Safety

Minimum Field Size at SAD : 3 x 3cm

Computerised motorized wedge

Asymmetric Collimator

Van de Graaff generator

Typically produces high

energy X-rays of 2MV

Can go upto 10MV but

limited by size and required

high-voltage insulation

No longer produced

commercially

Betatron

Principle- An electron in a

changing magnetic field

experiences acceleration in a

circular orbit

Can provide wide range of

energy from 6 MeV to 40 MeV

Dose rate and field size

capabilities of medical

betatrons are low compared

with LINACS and cobalt

machines

Betatron

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LINEAR ACCELERATOR

History 1948: A working 1MV linear accelerator was installed at the

Fermi Institute in Chicago.

1952: linac installed at Hammersmith Hospital in London.

1956: first clinically used in the US at the Stanford University

Hospital.

1961: The first 100 cm SAD fully isocentric linear accelerator

was manufactured and installed in the US

A 2-year-old boy suffering with

retinoblastoma was the first

patient to receive radiation from

the medical linear accelerator at

Stanford

History of LINAC in India

1976- 1st Linear accelerator set up in WIA Cancer Institute,Chennai

TMH got the 1st LA in 1980 –Mevatron 12.

1982 TMH got Clinac 6.

1994- 1st Dual energy LA 2100 C.

1999- 2nd dual energy LA 2100 CD.

2003- Single energy 6EX.

2005- Primus Dual Energy LINAC (ACTREC)

2007- Tomotherapy (ACTREC)

2009- Trilogy and Tomotherapy

2011- Novalis

Accelerator Generations

Early Accelerators (1953-1961):

Extremely large and bulky

Limited gantry motion

Second Generations (1962-1982):

360 degree rotational

Allow treatment to a patent from any gantry angle

Improvement in accuracy and dose delivery

Third generation accelerators:

Improved accelerator guide

Magnet systems

Beam-modifying systems to provide wide ranges of beam energy, dose rate, field size

First is the Power Supply which supplies power to

the modulator → Next comes magnetron or klystron

These devices convert the pulsed DC power into

microwave radiation that is supplied to the

Accelerator tube.

The modulator also supplies a signal to electron gun

which supplies the electrons to the accelerator

tube on command from the Modulator

Next is the wave guide system, these are copper

tubes that are used to efficiently transport

microwave energy from the magnetron or klystron

to the accelerator tube.

Basic Mechanism of LA

POWER

SUPPLYMODULATOR

MAGNETRON

OR

KLYSTRON

ELECTRON

GUN

Wave

Guide

System

Modulator

Power supply that converts AC to DC pulse forming network to

modulate current into pulses.

Pulse Forming Network(PFN) & Hydrogen Thyraton provide DC in

pulses.

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Thyratron

Gas-filled multielement tube

fabricated with glass or ceramic

tubes, metallic anodes, and one

or more grids.

Filled with hydrogen or

deuterium at low pressure.

Thyratrons are capable of

switching at rates greater than

20 kHz

The Magnetron

The cathode is heated by an inner

filament

Electrons are generated

by thermionic emission

Static M-field perpendicular to the plane of cavities

Electron move in complex spirals toward the resonant

cavities

Radiating energy in form of

microwave frequency of which is

about 3000MHz

Pulse E-field between cathode & anode

Electron accelerated toward the anode

The Klystron

Not a generator of microwaves

Microwave amplifier

Needs to be driven by a low-power microwave oscillator

The Klystron

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Electrons produced by the cathode

Electrons are accelerated by –ve pulse into buncher cavity

Lower level microwave set up an alternating E field across the buncher cavity

Velocity of e- is altered by the action of E-field (velocity modulation)

1. Some e- are speed up2. Other are slowed down

Passed in the drift

tube (field-free

space)

Electrons arrive catcher cavity

1. Generate a retarding E-field

2. Electrons suffer deceleration

3. KE of electrons converted into high-power microwaves

ACCELERATING WAVEGUIDE

The simplest kind of accelerating waveguide

is obtained from a cylindrical uniform

waveguide by adding a series of discs (irises)

with circular holes at the centre, placed at

equal distances along the tube.

These discs divide the waveguide into a

series of cylindrical cavities that form the

basic structure of the accelerating waveguide

in a linac.

Two types of accelerating waveguide have

been developed for the acceleration of

electrons:

TRAVELLING WAVEGUIDES

These guides have relatively low shunt

impedances compared to standing waveguide

systems therefore, they need to be physically

longer to achieve the same output energy.

Electrons from the gun end enter a velocity

of 0.8 c (at 80 kV) where c is the velocity of

light. After the first 30 cm, they are

travelling at velocities close to c.

This first part of the guide is called the

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Travelling wave guide

Riding the WavesAccelerators speed up charged particles by creating large electric fields

which attract or repel the particles. This field is then moved down the

accelerator, "pushing" the particles along.

Standing wave structure

In the standing wave structure each end of

the accelerating waveguide is terminated

with a conducting disc to reflect the

microwave power, resulting in a buildup of

standing waves in the waveguide.

In this configuration, at all times, every

second cavity carries no electric field and

thus produces no energy gain for the

electrons.

These cavities therefore serve only as

coupling cavities and can be moved out to

the side of the waveguide structure,

effectively shortening the accelerating

waveguide by 50%.

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Electron beam transport

In low energy linacs the target is embedded

in the accelerating waveguide and no beam

transport between the accelerating

waveguide and target is required.

Bending magnets are used in linacs operating

at energies above 6 MeV, where the

accelerating waveguides are too long for

straight-through mounting.

axis and the electron beam must be bent to

make it strike the X ray target or be able to exit

through the beam exit window.

● 90º bending (chromatic)

● 270º bending (achromatic);

● 112.5º (slalom) bending

Bending Magnet

Bending magnet: bends the electron beam through a right angle, so it ends up pointed at the patient

90 degree magnets (chromatic) have the property that any energy spread results in spatial dispersion of the beam.

Electrons are bent in proportion with their energy, the lower energy electrons are bent more, the higher energy electrons less

Results in a beam that is spread from side to side according to energy

Energy sensitive, act as energy differentiators

A 270o Magnet Using Hyperbolic

Pole Faces

more energetic electrons (larger radii) enter

closer spaced regions with higher bending

fields, and less energetic electrons encounter

lower bending fields between the wider pole

spaces.

all electrons that entered on axis at 0o

should converge again at the same point at

2700. The deflection is without dispersion

with energy (i.e. achromatic).

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Slalom Bending

➢used by Elekta (112.5 degree bend

➢solves energy dispersion and isocentric height problems

➢comprises three magnets: 2 at 45 degrees, one at 112.5

➢can focus to 2mm spot

Treatment Head

Linac in the photon mode –

start with the tungsten target

After the target will come the

primary fixed collimators which

determine the maximum extent of

the radiation field.

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There are 2 ion chambers in the

machine, one is used for backup in

case the other fails.

Radiation Output monitoring

Flatness and Symmetry monitoring

Sealed (reading independent of

temperature and pressure)

Same chamber for both photons

and electrons

It is the charge collected by the

chambers that

is used to shut down the machine

SCATTERING FOIL

A device to widen the thin pencil beam (3 mm) of electrons.

Metallic plates of tin, lead or aluminium are used.

Beam collimation can be improved by the use of dual foil system where the 1st filter spreads the beam out and the 2nd makes the dose homogenous by flattening are used.

Primary foil

Secondary foil

Electron

cone

Electron cone

Electrons readily interact with

the atmospheric air.

scattering of electron and beam

energy dissipation.

beam defining collimator with

variety of field sizes.

Secondary collimation close to

the patient’s body.

MLCs Type A

The Y jaws are replaced by bank

of MLCs.

Small traveling range of leaves

Shorter leaf length & compact

head diameter.

Used by Elekta

MLCs type B

The lower jaws i.e. X jaws are

replaced with bank of MLCs.

Used by Siemens

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MLCs Type C

MLC placed just below standard

upper & lower jaws.

Tolerance on leaf positioning &

leaf dimension is relaxed.

collimator will have over bulk.

Treatment Head & patient

clearance is considerably

reduced.

Used by Varian

Auxiliary system

The linac

auxiliary

systems:

Photo of Tomotherapy Unit with covers

removed

Forward Planning : Physicist specify Beams weights, Wedge angles, beam modifiers. They have direct control over these parameters at any point in the treatment planning process.

Inverse planning : Physicist does not specify these parameters. The numbers, energy & direction of the beams are still chosen by the physicist. But once beams have been specified, the computer takes complete control over all parameters e.g.MUs, weights, beam modifiers etc.

Item 2100CD 2100C Clinac 6EX

Photon Energy 6 MV, 15 MV 6MV, 10 MV 6 MV

Electron 6,9,12,15, 18MeV 6,9,12,16,20MeV X

Wedges Dynamic wedge option “ “

Field Size( with

wedges)

Max: 40x40cms

Min .5x.5

“

“

“

“

Dose Rate 100-600 MU/Min 80-400 100-600

Wedges 15,30 Steel

Rest Lead

15, 30 Steel

Rest Lead

15, 30Steel

Rest Lead

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Trilogy Novalis Tomotherapy

Wave guide Standing Standing Standing

Length 140cm 140cm 140cm

Rf power source

Klystron Klystron Magnetron

Rf frequency 3000Mhz 3000Mhz 3000Mhz

Field size Min-0.5 x 0.5Max-40 x 40cm

Min-0.5 x 0.5Max-40 x 40cm

Min-40 x 1cmMax-40 x 5cm

Jaws Asymmetric Asymmetric Symmetric

MLC 120(60 pairs) 120(60 pairs) 64 binary

Wedges 15,30 Steel 15,30 steel NA

COBALT LA

DOSE RATE Decreases constant

Energy source

diameter(penumbr

a)

high less

Source change 12,000 curie source= 35

lakhs

none

PDD at 10cm 55% 67%

Minimum field size 5x5cm 0.5x0.5 cm

Motorized wedges Not possible possible

Asymmetric jaws Absent present

COBALT LA

Source disposal Major problem nill

Cost of machine 1.9 crores 3 crores

True beam

Product of Varian

Flattening filter free

X ray(MV) – 4,6,8,10,15,18,20

Electrons (MeV) –

6,9,12,15,16,18,20,22

Max dose rate – 1000MU/min

Millenium 120 leaf MLC,

Max field size – 40 x 40cm

OBI -KV CBCT

Cyclotron

- Used as source of high energy

protons/neutron

-Consist of 2 electromagnets

-Hollow copper D

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LINAC CYCLOTRON

Large space requirement (few hundred m

long) but lightCompact but heavy

Expensive Cheaper in construction

Less efficient power conversion More efficient power conversion

Upgradable in energy Difficult to upgrade in energy

Straightforward beam extraction Difficult extraction and related beam losses

Capable of high beam current (100 mA) Modest beam current capability (5 mA)

Synchrotron

IN CYCLOTRON-As momentum and energy increase and the velocity of a particle approaches that of light, then the velocity begins to increase less rapidly than the particle mass, so the revolution frequency drops so that particles are no longer synchronous with the accelerating potential.

- IN SYNCHROTRON

A short pulse of particles is injected at low magnetic field, the field rises in proportion to the momentum of particles as they are accelerated and this ensures that the radius of the orbit remains constant.

The accelerated particles take less and less time to complete their orbit so the frequency of the accelerating alternative current must increase as well.

Synchrotron

Notice:

particles

travel in

a

ring, not

a disk

GAMMA KNIFE

1951 - Lars Leksell introduced the concept of

radiosurgery

System basically consists of the radiation unit

with the patient couch and collimator helmet,

the control panel and auxiliary systems.

Radiation System

▪ 201 cobalt-60 sources arranged in a hemispherical

pattern

▪ Each source is mounted at the head of a collimator

channel

which guides the radiation from the source to the focus

point

▪ Total activity of approximately 2.22x1014 Bq.

Shield assembly sources are heavily shielded within a cast iron body with

shielding doors

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CyberKnife Robotic Radiosurgery

System

Continuous feedback

Automated system

Detects any movement in tumor

or patient location

Corrects to ensure sub-millimeter

accuracy

Clinical Benefits

Staged/Fractionated Radiosurgery

1-5 fractions/stages

Larger lesions

Lesions next to critical structures/organs at risk

Improved Patient Quality of Life

Short treatment course: 1-5 days CyberKnife vs. 6-8 wks

Radiotherapy

Optimal for patients

Optimal for patients with limited life expectancy

Increased convenience

No infection risk

THANK

YOU

Dr Ashutosh

Dr

Carthikeyan

Dr Jatin